Crown gears, also known as face gears, are gears that are used in angle drives. Crown gears generally have teeth that are radially extending in their longitudinal direction with an increasing pressure angle from the inside end to the outside end of each tooth.
Crown gears mesh with a cylindrical pinion having straight or helical toothing. Usually, the axes of the crown gear and pinion intersect at an angle of ninety degrees with one another but this is not always the case. The angle of intersection may be other than ninety degrees, or, the axes may cross but not intersect one another. Gear sets comprising a crown gear and its mating pinion have advantages such as the absence of any need for axial adjustment of the pinion, a high gear ratio, and a high transverse contact ratio being achievable without special provisions.
A method of manufacturing crown gears is taught by U.S. Pat. No. 2,304,586 to Miller. A hobbing method is disclosed wherein the form of the hob is based on the tooth form of a pinion which meshes with the desired crown gear. The shape of the cutting edge of the hobbing tool, as seen in an axial section, is always the shape of the outer contour of a cross-section of a segment of the pinion. As the hob rotates, its helically arranged teeth emulate the movement of the teeth of the pinion turning in mesh with the crown gear.
Several methods exist for finishing crown gears. One such method is disclosed in U.S. Pat. No. 4,954,028 to Seroo et al. and comprises a grinding disc having an outer grinding periphery corresponding to the tooth profile of a mating pinion to the crown gear.
U.S. Pat. No. 2,309,530 to Perkins teaches positioning a crown gear into meshing engagement with two pinions, one pinion driving the crown gear which in turn drives the second pinion. While the gears are rotating, the crown gear is caused to traverse the pinions. The teeth of the crown gear are shaped by the form of the pinion teeth and are finished by abrasive or cutting action or by metal displacement depending on the material of the crown gear and pinions.
In a similar manner, published PCT International application No. WO 92/18279 teaches a method for finishing hardened crown gears by meshing the crown gear with a honing pinion while moving the gear and pinion relative to one another back and forth along the axis of the honing tool.
Published PCT International application No. WO 92/09395 discloses a grinding wheel for finishing crown gears which is formed based on the previously mentioned U.S. Pat. No. 2,304,586 in that the helically arranged cutting elements are essentially replaced with a grinding thread or rib helically arranged about the periphery of the tool. This type of tool is known as a hobbing grinding disc or threaded grinding wheel.
The threaded grinding wheel of WO 92/09395 is formed with the ribs being free of undercuts such that the grinding wheel may be dressed without the cross-section of the ribs being changed as a result of the dressing. Dressing of the grinding wheel surfaces takes place in dressing tool increments of 1-5 degrees between a minimum angle and a maximum angle based on the minimum and maximum pressure angles for a crown gear to be produced. This incremental dressing process, however, results in rib profile surfaces that are not smooth but instead consist of a series of flat portions due to the incremental positions of the dressing tool.
In published PCT International Application No. WO 92/11967, a hobbing grinding disc is utilized for grinding crown gears. The grinding wheel is dressed by a dressing tool having the same shape as the crown gear to be ground. While the dressing tool and grinding wheel are rotating in mesh, additional relative motion is included along the length and height of the teeth of the dressing tool to ensure that all surfaces of the grinding wheel are contacted by the dressing tool. Dressing tools of this type are complex in that accurate diamond deposition on the tooth surface geometry of the crown gear shaped dressing tool is difficult to control.
It is an object of the present invention to provide a dressing process for a threaded grinding wheel for the grinding of crown gears wherein a smooth, accurate grinding profile surface is formed with a simple dressing tool.